The development and maintenance of granular convoluted tubule cells in the mouse submandibular gland (SMG) and the production of renin-1, renin-2, and epidermal growth factor (EGF) by these cells are under complex hormonal control. Hypophysectomy causes profound involution and loss of renin activity in this gland. We have shown previously that T4 acts synergistically with 5a-dihydrotestosterone (DHT) to restore SMG morphology and renin-2 activity in hypophysectomized female mice. Investigating the mechanism of T4 and DHT interaction in the hypophysectomized mouse proved impractical, and in the present study we have used genetically hypothyroid (hyt/hyt) mice that carry the structural gene for renin-1 but not for renin-2. Levels of SMG renin-1 and EGF in hyt/hyt mice were less than 4% of those in euthyroid (hyt/+) littermates. Administration of a pharmacological dose of T4 (2.5 βg/g BWday, ip) to male hyt/hyt mice for 18 days restored SMG renin-1 and EGF to near-normal levels. The weights of SMG, seminal vesicle, and epididymis were also lower in hypothyroid mice and increased in response to T4. The effect on SMG renin-1 and EGF of either DHT (150 βg/g BW every other day, sc) or T4 (0.025-2.5 βg/g BW-day, ip) was blunted in female hyt/hyt mice. A combination of DHT and T4 (0.1 βg/g BWday) that restored total circulating T4 and T3 to physiological levels acted synergistically to increase SMG renin-1 and EGF. The administration of 2.5 βg T4/g BWday plus DHT for 7 days increased the specific activity of SMG renin-1 and EGF to levels approaching those in euthyroid littermates given the same treatment. T4 (0.1 βg/g BW-day) did not alter the quantity or sedimentation characteristics of high affinity androgen-binding protein in SMG from female hyt/hyt mice and induced SMG renin-1 in Tfm/Y mice. Thus, T4 does not appear to exert its effect via the androgen receptor. The administration of DHT and T4 to female hyt/hyt mice produced lower circulating levels of both T3 and T4 than the same dose of T4 given alone, suggesting that DHT does not act by enhancing the conversion of T4 toT3. This study demonstrates that the interaction of T4 and DHT is not a pharmacological phenomenon, but occurs at doses of T4 that restore serum T3 and T4 in female hyt/hyt mice to normal or near-normal levels. Furthermore, since T4 and DHT act synergistically to increase the levels of at least three SMG proteins, these hormones must interact at some level that influences the expression of multiple genes. This conclusion is supported by morphometric data showing that a physiological dose of T4 in combination with DHT also enhances the differentiation of granular convoluted tubule cells.
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